1. Field of the Invention
The invention relates to organic light emitting diode (OLED) displays.
2. Description of the Related Art
OLED displays require no backlight, and are therefore optimum for thin formation, with no limitation of viewing angle. Thus, OLED displays have become popular substitutes for cathode ray tube (CRT) and liquid crystal display (LCD) devices.
One problem using organic light emitting elements is a mura defect, which is caused mainly by inconsistent luminance of the organic light emitting elements. Luminance of an organic light emitting element is determined in a manufacturing process and degrades with time. The rate of luminance decay of an organic light emitting element depends especially on characteristics of the organic light emitting element, conditions in a manufacturing process, how the organic light emitting element is driven, and other conditions.
Mura defects can be aggravated in full-color OLED display panels that emit red, green, and blue light. The organic light emitting elements of varying colors have different rates of luminance decay. The differences in luminance between the organic light emitting elements of a plurality of colors typically become more apparent with time.
FIG. 1 as disclosed in U.S. Pat. No. 6,710,548, depicts a display panel. A pixel array 102 of the panel has a plurality of pixels 104, each with an organic light emitting element 110. A video signal is written to the pixels by controlling a source line driver 106 and a gate line driver circuit 108. A current value (measured value) of total current through all pixels is measured by an ammeter 114. A correction circuit 116 controls a variable power supply 112 to compensate for the difference between the measured current and a reference value calculated from the video signal. Light emitted by the organic light emitting elements, however, cannot be corrected individually. Once the output of the variable power supply 112 is changed, driving signals (current or voltage) that drive the organic light emitting elements are all changed.